bpo-32030: Split Py_Main() into subfunctions (#4399)

* Don't use "Python runtime" anymore to parse command line options or
  to get environment variables: pymain_init() is now a strict
  separation.
* Use an error message rather than "crashing" directly with
  Py_FatalError(). Limit the number of calls to Py_FatalError(). It
  prepares the code to handle errors more nicely later.
* Warnings options (-W, PYTHONWARNINGS) and "XOptions" (-X) are now
  only added to the sys module once Python core is properly
  initialized.
* _PyMain is now the well identified owner of some important strings
  like: warnings options, XOptions, and the "program name". The
  program name string is now properly freed at exit.
  pymain_free() is now responsible to free the "command" string.
* Rename most methods in Modules/main.c to use a "pymain_" prefix to
  avoid conflits and ease debug.
* Replace _Py_CommandLineDetails_INIT with memset(0)
* Reorder a lot of code to fix the initialization ordering. For
  example, initializing standard streams now comes before parsing
  PYTHONWARNINGS.
* Py_Main() now handles errors when adding warnings options and
  XOptions.
* Add _PyMem_GetDefaultRawAllocator() private function.
* Cleanup _PyMem_Initialize(): remove useless global constants: move
  them into _PyMem_Initialize().
* Call _PyRuntime_Initialize() as soon as possible:
  _PyRuntime_Initialize() now returns an error message on failure.
* Add _PyInitError structure and following macros:

  * _Py_INIT_OK()
  * _Py_INIT_ERR(msg)
  * _Py_INIT_USER_ERR(msg): "user" error, don't abort() in that case
  * _Py_INIT_FAILED(err)
diff --git a/Python/bootstrap_hash.c b/Python/bootstrap_hash.c
index 807d602..e667fa9 100644
--- a/Python/bootstrap_hash.c
+++ b/Python/bootstrap_hash.c
@@ -561,15 +561,16 @@
     return 0;
 }
 
-static void
+static _PyInitError
 init_hash_secret(int use_hash_seed,
                  unsigned long hash_seed)
 {
     void *secret = &_Py_HashSecret;
     Py_ssize_t secret_size = sizeof(_Py_HashSecret_t);
 
-    if (_Py_HashSecret_Initialized)
-        return;
+    if (_Py_HashSecret_Initialized) {
+        return _Py_INIT_OK();
+    }
     _Py_HashSecret_Initialized = 1;
 
     if (use_hash_seed) {
@@ -593,12 +594,14 @@
            pyurandom() is non-blocking mode (blocking=0): see the PEP 524. */
         res = pyurandom(secret, secret_size, 0, 0);
         if (res < 0) {
-            Py_FatalError("failed to get random numbers to initialize Python");
+            return _Py_INIT_ERR("failed to get random numbers "
+                                "to initialize Python");
         }
     }
+    return _Py_INIT_OK();
 }
 
-void
+_PyInitError
 _Py_HashRandomization_Init(_PyCoreConfig *core_config)
 {
     char *seed_text;
@@ -608,13 +611,13 @@
     if (use_hash_seed < 0) {
         seed_text = Py_GETENV("PYTHONHASHSEED");
         if (Py_ReadHashSeed(seed_text, &use_hash_seed, &hash_seed) < 0) {
-            Py_FatalError("PYTHONHASHSEED must be \"random\" or an integer "
-                          "in range [0; 4294967295]");
+            return _Py_INIT_USER_ERR("PYTHONHASHSEED must be \"random\" "
+                                     "or an integer in range [0; 4294967295]");
         }
         core_config->use_hash_seed = use_hash_seed;
         core_config->hash_seed = hash_seed;
     }
-    init_hash_secret(use_hash_seed, hash_seed);
+    return init_hash_secret(use_hash_seed, hash_seed);
 }
 
 void